(Supported by United States-Israel Binational Science grant #95-00350/2 to J. Orly) Steroidogenic mitochondria of adrenal cortical fasciculata tissue are unique in the animal kingdom. Conventional TEM studies have shown that the cristae membranes of such mitochondria appear as multiple vesicles instead of the common elongated-plate or tube found in other cell types. However, it is unlikely that such cristae vesicles are physically separated from the circumferential inner mitochondrial membrane adjacent to the outer membrane. Previous TEM studies had suggested that each vesicle is connected to the space between the inner and outer membrane by a thin stalk-like tube. Electron tomography was used to investigate the inner mitochondrial membrane architecture in three dimensions. A double-tilt tomographic reconstruction of a 0.5(m section was made using the IVEM, and the images were electronically sent to Dr. Orly. A second double-tilt reconstruction was made of a 0.25(m section. Selected portions of the reconstruction were traced with Sterecon, a surface-rendered model was created, and a videotape of movies from the data was made. All the data was sent to Dr. Orly on CDROM, and the video was shown during Dr. Orly's seminar at the Wadsworth Center. The conclusion so far is that the cristae form both individual vesicles and groups of parallel long, straight tubes with occasional interconnections to each other. Connections to the intramembrane space can be found, but the overall connectivity of the membranes has not yet been established in detail.